1. Field of the Invention
This invention relates generally to medical apparatus and methods, and more specifically to implants and biodegradable implants for use in the vascular system as well as other body lumens and cavities.
The use of implants in body tissue is becoming increasingly important in medical treatment. Examples of implant usage include alteration of tissue in cosmetic or reconstructive procedures such as breast augmentation as well as creation, preservation or closure of lumens, channels or fluid reservoirs (e.g. stenting stenotic lesions, exclusion of aneurysms or embolic coils). Implants are also used as matrices for tissue growth (e.g. orthopedic bone fusion procedures), to control unwanted tissue growth and for delivery of therapeutic agents to tissue. Implants may also be employed to join tissue surfaces together or for isolating or protecting tissue lesions in order to enable or mediate healing. Implants are also used to mediate the rate of substances or energy passing into, out of, or through tissue.
Often, implants are fabricated using various metals and/or polymers. Examples of common metals include stainless steel, titanium, nickel-titanium alloys like Nitinol and polymers such as PTFE (e.g. Teflon®), polyethylene, polyurethane and polyester are often used in implants. A potential disadvantage of these permanent implants is that the implant materials may be harder and stiffer than the surrounding tissues, thus anatomical or physiological mismatch may occur, potentially resulting in tissue damage or causing unwanted biological responses. Some materials may fatigue over time and break which can disrupt the layer of endothelial cells potentially causing thrombosis. Additionally, a permanent implant is not always required. An implant may only be required for a limited time period, therefore the implant often must be surgically explanted when it is no longer needed. To overcome some of these challenges, the use of biodegradable polymeric implants has been proposed. Examples of implantable biodegradable polymers include the aliphatic polyester polylactic acid or polylactide (PLA) and polyglycolide (PGA). PGA was originally proposed for use in suture material in the late 1960's. By the early 1970's PLA was proposed as a suture material including both the optically active poly-L-lactide (PLLA) and the racemic mixture poly-DL-lactide (PDLA). PLLA has also been used in biodegradable stents, as reported by Igaki and Tamai. A co-polymer of PLA and PGA, known as PLGA has also been proposed for use in implants. Another material which has recently been proposed (in the 1980's) for use in sutures and orthopedic implants is polydioxanone. In the mid-1990's implantable drug delivery systems using polyanhydrides were proposed by Langer et al. at the Massachusetts Institute of Technology, and more recently tyrosine derived polyarylate has seen use in hernia repair and companies are developing biodegradable stents composed of materials such as a tyrosine derived polycarbonate, poly(DTE carbonate).
While these newer biodegradable implant materials have overcome some of the challenges of earlier implant materials, other potential drawbacks still exist. For example, it is often desirable to adjust the shape of some implants in situ so that the implant conforms more accurately to the anatomy of the treatment site. However, the biodegradable polymers cannot be plastically deformed, molded or shaped at normal body temperatures since they must be solid at body temperature. The implant must therefore be heated above its glass transition temperature, Tg. Often the glass transition temperature is fairly high, for example PDLLA and PLLA have a Tg approximately 50°-80° C., therefore in situ heating may result in localized tissue damage, thrombosis or patient discomfort. It is well known that adding an impurity to a material will change some of the material's properties such as increasing its boiling point and reducing its freezing point. Therefore, additives may be mixed with the biodegradable polymers to decrease the glass transition temperature, for example 2-10% ε-caprolactone added to 90-98% PLLA can reduce the glass transition temperature down to about 38°-55° C., but a heat source hotter than the glass transition temperature may still be required due to heat transfer inefficiencies or non-uniform heating, therefore, similar complications may still arise.
One proposed solution to the challenge of non-uniform heating is to coat the implant with a radiation absorbing material which converts radiation to heat. Exemplary coatings include chromophores like indocyanine green, vital blue, carbon black and methylene blue. The radiation, often ultraviolet or visible light must therefore be supplied in situ from a second device due to the poor penetration of the radiation through the tissue. Additionally, production of sufficient and uniform heat using this technique remains a challenge. Furthermore, the chromophores may degrade into unwanted chemicals that are toxic to the body. Therefore, there exists a need for an easier, less toxic and less invasive way to heat implants, including biodegradable polymer implants, to an elevated temperature so that they may be shaped or molded in situ. Furthermore, such techniques should also be able to heat the implant uniformly.
Additionally, while biodegradable implants will degrade over time, it would also be desirable to be able to control the rate of degradation. For example, when an implant is no longer required, it would be desirable to be able to accelerate the degradation rate so that the implant breaks down faster than its normal in situ rate. For this reason, there is also need for a way to control the degradation rate of a biodegradable implant.
2. Description of the Background Art
Prior patents describing nanoshells for converting incident radiation into heat include: U.S. Pat. Nos. 6,344,272; 6,428,811; 6,530,944; 6,645,517; 6,660,381; 6,685,730; 6,699,724; 6,778,316; and 6,852,252. Prior patents describing thermo-mechanically expansion of stents include: U.S. Pat. Nos. 5,670,161; 5,741,323; 6,607,553; 6,736,842. Prior patents describing meltable stents include: U.S. Pat. Nos. 4,690,684 and 4,770,176. Prior patent describing bioerodable polyanhydrides for controlled drug delivery include: U.S. Pat. No. 4,891,225. Prior patents describing tyrosine derived polycarbonate as an implant include: U.S. Pat. Nos. 6,951,053; 7,101,840; and 7,005,454. Prior patents describing biodegradable stents include: U.S. Pat. Nos. 5,733,327; 5,762,625; 5,817,100; 6,045,568; 6,080,177, 6,200,335; 6,413,272; 6,500,204; 6,632,242; RE38,653; RE38,711; 7,066,952; and 7,070,615. The full disclosure of each of these patents is incorporated herein by reference.